The present invention relates to a magnetic disk device, and is specifically suited to a magnetic disk device, in which a rotary section including a shaft is supported through a fluid bearing.
In magnetic disk devices for recording/reproduction of data, there is a need for a large volume of recorded data and for an increase in data transmission speed by virtue of performing a work for recording/reproduction of data at high speed. In order to meet the need, it is effective to increase the number of revolutions of a magnetic disk to reduce time for access to a data storage location. As magnetic disk devices used as a server, models, in which a magnetic disk makes 10,000 revolutions per minute, constitute the mainstream while high-speed rotating machines making 15,000 revolutions per minute have appeared in recent years and high-speed rotation of a magnetic disk has become the trend in the market.
When a magnetic disk is rotated at high speed, however, there are increased frictional torque called windage loss between the magnetic disk and air and frictional torque of a bearing called bearing loss. Therefore, it is necessary to supply much torque to a spindle motor, which leads to an increase in electric power consumption. An increase in electric power consumption brings about large electric current and temperature rise in an associated device, for which there is tolerance determined by specifications of parts in the device, so that it becomes difficult to increase electric power consumption beyond a conventional one. In, for example, high-speed rotating machines, in which a magnetic disk makes 15,000 revolutions per minute, the diameter of the magnetic disk is decreased to 65 mm from 84 mm of a conventional one to reduce windage loss to restrict an increase in electric power consumption. When a magnetic disk is made small, it is reduced in recording capacity, so that in order to attain high-speed rotation further in the future, other techniques for reduction in electric power consumption than making a magnetic disk small in diameter are demanded.
On the other hand, in magnetic disk devices of recent years, the demand for small vibration and low noise begins, instead of ball bearings the use of dynamic oil pressure bearings, which use oil for a working fluid, as bearings for spindle motors (for example, JP-A-2000-306319 (Patent Document 1)). Since fluid bearings involve generation of frictional loss called bearing loss between oil and a sleeve made of metal or the like, it is also important to reduce bearing loss in rotating a magnetic disk at high speed. In particular, since a ratio occupied by bearing loss in total electric power consumption increases when a magnetic disk is made small to reduce windage loss, reduction in bearing loss contributes much to reduction in electric power consumption.
For reduction in bearing loss, it is effective to make a bearing small in diameter to reduce a frictional surface. In order to achieve this, it is effective to make a shaft, that is,a rotating shaft of a spindle motor, on which a bearing is mounted, small in diameter. Since the diameter of a shaft has an effect of the fourth power thereof on stiffness of the shaft, however, there is a disadvantage that making the diameter small leads to much reduction in stiffness of the shaft to cause elastic deformation of the shaft, so that vibration accompanied by falling of the shaft becomes large. When a shaft falls, a head is worsened in positioning accuracy to make high recording density difficult, so that it is important to suppress shaft falling vibration. In recent years, there is a tendency to the use of a simple rotating shaft construction due to the demand for reduction in cost, in which case a cantilever arrangement with only one end of a shaft fixed is used, so that stiffness is small and vibration accompanied by falling of the shaft becomes further large.